ALUMINUM. 727 



The most valuable of the alloys are those with copper. Aluminum 

 brouze has great tensile strength. A bar containing 11 per cent, alumi- 

 num made by the electric furnace and tested by the Leeds Forge Com- 

 pany, limited, gave a tensile strain of 57,27 tons, or 128,400 pounds to the 

 square inch. One, containing 7.5 per cent, aluminum, tested by Professor 

 Unwin, broke under 3G.78 tons = 89,743 pounds to the square inch. In 

 resistance to compression this alloy equals the best steel; its transverse 

 strength, or rigidity, is about forty times greater than ordinary brass. 

 Its elastic limit is higher than that of mild steel, and it can be worked 

 at a bright-red heat as easily as wrought iron. Its mechanical and 

 physical properties render it useful for every variety of metal work, its 

 high price only having hitherto restricted its use. Its enormous 

 strength and anticorrodible qualities recommend it as valuable above 

 any other alloy for propeller blades, stern and rudder frames, and for 

 hydraulic and engineering work generally. With above 11 per cent, the 

 alloy becomes brittle ; and at 20 per cent, can be powdered readily in a 

 mortar. The addition of small quantities of aluminum lowers the fus- 

 ing point of iron, and this is utilized in the " Mitis " castings. It insures 

 freedom from blow-holes, increased tensile strength, and high elastic 

 limit. Mr. Keep found that 0.1 per cent, aluminum raised the transverse 

 breaking strength of a one-half inch bar, 13 inches long, from 379 pounds 

 to 545 pounds, or 44 per cent., and the resistance to impact from 239 

 pounds to 254 pounds, or G per cent. The tensile strength of Mitis cast- 

 ings may be as high as 27 tons pf^r square inch, with an elongation of 20 

 per cent. Another alloy made in the electric furnace is silicon bronze, 

 which, owing to its great strength and tenacity, its resistance to corro- 

 sion, combined with high electrical conductivity, is perhaps the best 

 metal extant for electric light, telephone, and telegraph wires. 



